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West Elk Mine <br />• caved zone heights closer to 4t. An acceptable average value for the Apache Rocks and Box <br />Based on the stratigraphic and lithologic information obtained from drill holes in the South of <br />Divide mining area, the rocks consist of a greater amount of shales, siltstones, and claystones <br />than are present in the Apache Rocks and Box Canyon mining areas. It is therefore estimated <br />that the caved zone will range from 2t to 5t, depending on water conditions encountered and on <br />specific roof lithology. In a dry environment, where lenticular sandstones comprise the E Seam, <br />roof, the caved zone will be closer to 2t. In a wet environment where soft shales and claystones <br />occur in the roof, however, the caved zone will likely be closer to 5t. <br />Fractured Zone <br />A zone of fracturing and local separation along rock bedding planes and joints occurs above the <br />zone of caving. In this zone, which is transitional to the underlying caved zone, lateral and <br />vertical constraints in the adjacent overburden strata and the caved rocks below minimize further <br />displacement or rotation of the fractured rock. Displacements in the fractured zone and severity <br />of fracturing tend to decrease upward as lateral and vertical confining stresses increase. <br />Based on width and conductivity of fractures Peng (1992, p. 143) states that the upper one-third <br />of the fractured zone (in terms of height) has only minor fractures with little potential for water <br />conductivity. In the lower two-thirds of the fractured zone, water conductivity increases <br />progressively downward. <br />Compression arches (arcuate zones of compressive stress) commonly develop, or partially develop, <br />above the mining panels. These arches temporarily transfer overburden stresses to the panel barrier <br />or chain pillars and also to the caved zone and the mining face (Dunrud 1976). The arches in a <br />given area commonly move upward and disperse as longwall mining is completed in the area.' <br />Compression arches may not disperse where the room-and-pillar mining method is used, because <br />pillars and stumps left after mining may prevent dissipation of the arches. The rocks affected by the <br />arches temporarily are subjected to increased stress and strain as the arches move upward. <br />However, in the longwall mining area, this increased stress and strain commonly is less than it is in <br />room-and-pillar mining areas because stresses are relieved as the arches move upward and dissipate. <br />Peng (1992) reports that the combined height of the zone of caving and fracturing ranges from 20 <br />to 30 extraction thicknesses (20 to 30t), and that the height.of the fractured zone is greater for. <br />hard, strong rocks than it is for soft, weak rocks. <br />The l1eiglzt of the zone of fracturing is a ar?ction of lithology and layer thickness, according to Peng <br />(19921'. For example, the zone of fracturing commonly is higher for strong, thickly-bedded, brittle <br />sandstones than it is for thinly layered, sots, plastic shales and claystones. Liu (1981), reports <br />ra?-:•aes of hClghts of the zone of frac a..V for various rock tN-pes as follows: <br />iei`las of 20 to 30 times coal e:itraenon rlicKrl '20 to 30t) are reported In strong brittle <br />rocks, such as siliceous sandstones and Iimestones, a value of 28t was reported for overburden <br />containing 70 percent sandstone. Also, because of hardness, fractures do not close as readily in <br />brittle rocks as they do in soft rocks during recompression <br />Canyon mining areas is 2.St. <br />2.05-124 Revised June 200 PRIO, Rev. Alarclt 2006; A9ap 2006 PRIG, Nov. 2006TR107;Apr-i12007TR108;Sep. 2007 PR12; Feb. 2008 PR-12